Method for Producing a Doped Organic Semiconducting Layer

ABSTRACT

A process is provided for producing a doped organic semiconductive layer, comprising the process steps of A) providing a matrix material, B) providing a dopant complex, and C) simultaneously applying the matrix material and the dopant complex to a substrate by vapor deposition, wherein, in process step C), the dopant complex is decomposed and the pure dopant is intercalated into the matrix material.

The invention relates to a process for doping an organic semiconductivelayer and to a process for producing an optoelectronic device which hasa charge transport layer.

This patent application claims the priority of German patent application10 2008 011 185.6, the disclosure-content of which is herebyincorporated by reference.

Doping of organic semiconductive layers may be advisable, for example,in optoelectronic devices such as organic light-emitting diodes in orderto improve the charge carrier injection or the charge carrier transportin the layers. Conventional processes for doping an organicsemiconductive layer lead to dopings which are inhomogeneous and/ordiffuse within the layer and hence adversely affect both the lifetime ofthe optoelectronic devices and the reliability of the charge carrierinjection. Furthermore, existing methods are inconvenient and costly.

It is an object of the invention to provide a process for producing adoped organic semiconductive layer which is inexpensive and in whichmanageable materials are used. This object is achieved by a processaccording to claim 1. A process for producing an optoelectronic devicewhich has a charge transport layer which is produced by the process forproducing a doped organic semiconductive layer is specified in claim 12.Further embodiments of the processes are the subject of further claims.

In one embodiment, a process for producing a doped organicsemiconductive layer is specified, which has the process steps of A)providing a matrix material, B) providing a dopant complex, and C)simultaneously applying the matrix material and the dopant complex to asubstrate by vapor deposition. In process step C), the dopant complex isdecomposed and the pure dopant is intercalated into the matrix material.In this case, a readily manageable, especially a volatile and readilyevaporable, dopant complex is selected, which, on decomposition,releases the pure dopant which is then intercalated directly into thematrix material.

In process step A), a matrix material can be selected from a groupcomprising phenanthroline derivatives, imidazole derivatives, triazolederivatives, oxadiazole derivatives, phenyl-containing compounds,compounds with fused aromatics, carbazole-containing compounds, fluorenederivatives, spirofluorene derivatives and pyridine-containingcompounds.

Examples of matrix materials which can be provided in process step A)are given hereinafter. A phenanthroline derivative may, for example, beBphen (4,7-diphenyl-1,10-phenanthroline, formula 1)

or BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, formula 2).

Examples of imidazole derivatives are TPBi(1,3,5-tris-(1-phenyl-1H-benzimidazol-2-yl)benzene, formula 3)

and compounds similar to TPBi.

One example of a triazole derivative is TAZ(3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole, formula4).

Oxazole derivatives are, for example,Bu-PBD((2-4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole).

Phenyl-containing compounds and compounds with fused aromatics are, forexample, DPVBi (4,4′-bis(2,2-diphenylethen-1-yl)diphenyl), rubrene,α-NPD (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine), 1-TNATA(4,4′,4″-tris(N-(naphth-1-yl)-N-phenyl-amino)triphenylamine).

Carbazole-containing compounds may be BCzVBi(4,4′-bis(9-ethyl-3-carbazovinylene)1,1′-biphenyl), but also smallercarbazole derivatives, for example CBP(4,4′-bis(carbazol-9-yl)biphenyl), which can form complexespredominantly via the π system.

In addition, the matrix materials used may also be bipyridyl-,terpyridyl- or tripyridyl-containing compounds, and stronglyelectron-withdrawing substances, for example F₄TCNQ(tetrafluorotetracyanoguinodimethane).

In addition, in the process, in process step C), the dopant complex maybe decomposed to a dopant and at least one ligand. Dopant complexessuitable for the process are those which are evaporable and candecompose in the process, for example complexes with gaseous or volatileligands.

The dopants selected may additionally be metals and/or metal clusters.The metals may be selected from a group comprising transition metals,lanthanoids and metals of the main groups. It is thus possible to use,for example, Zn, Cd, Hg, Mg, Ca, Sr, Ba, Al, Ga, In, Ti, Bi, Sn, Pb, Fe,Cr, Co, Os, Ru, Rh, Ir, Ni, Cu, Mn, Re, W, Mo, Nb, Zr, As, Sb, V, Ta,Ti, Sc, and, as lanthanoids, for example, Ce, Er, Gd, Hf, La, Nd, Pr,Sm, Tb, Tm, Yb.

The ligand selected in the process may be a ligand from a groupcomprising carbonyl, phosphine, cyclopentadienyl and arene ligands.These ligands are volatile and can be eliminated from the dopant in thegas phase under the influence of energy, such that the dopant, forexample the metal atoms and/or metal clusters, is obtained in pure form.Table 1 gives examples of dopant complexes with illustrative transitionmetals.

TABLE 1 Metals/Metal clusters Example of dopant complex Transition group3 Cp₃Sc Transition group 4 Cp₂Ti(CO)₂, Cp₂Ti(acetylene derivatives),Cp₂Zr(acetylene derivatives), Cp₂Hf(acetylene derivatives) Transitiongroup 5 V(CO)₆, V(Cp)₂, NbCp₄, TaCp₄ Transition group 6 Cr(CO)₆,dibenzenechromium, Mo(CO)₆, W(CO)₆ Transition group 7 Mn₂(CO)₁₀,Re₂(CO)₁₀ Transition group 8a Fe(CO)₅, Fe₂(CO)₉, Fe₃(CO)₁₂, ferrocene,Ru(CO)₅, Ru₃(CO)₁₂, Ru₆(CO)₁₈, Os(CO)₅, Os₃(CO)₁₂ Transition group 8bCo₂(CO)₈, Co₄(CO)₁₂, Co₆(CO)₁₆, Rh₂(CO)₈, Rh₄(CO)₁₂, Rh₆(CO)₁₆,Ir₄(CO)₁₂, Ir₆(CO)₁₆ Transition group 8c Ni(CO)₄ Transition group 1bCyclopentadienyl(triethylphosphine)- copper(I)

It is additionally possible to provide, through the reaction ofcarbonylmetallate anions and carbonylmetal halides, mixed metalcarbonyls, for example (OC)₄Co—Pt(py)₂-Co(CO)₄, H₃ReOs₃(CO)₁₂ andHCoRu₂(CO)₁₃. Some or all of the CO ligands may also be replaced byphosphine ligands. This widens the selection of metals in the transitionmetal series which can be used in the process as a dopant. Thecyclopentadienyl or arene ligands may also be substituted in order thusto adjust the evaporation and decomposition properties of the dopantcomplexes.

Zn can be used as a dopant complex, for example, in the form of Cp*₂Znor Zn(alkyl)₂ where alkyl=methyl or ethyl.

Main group elements such as Mg, Ca, Sr and Ba can be complexed in theform of Cp₂Mg, Cp*₂Mg, Cp₂Ca, Cp₂Sr and Cp₂Ba.

Individual Al, Ga, In, Tl and Bi atoms can be used in the process asdopant complexes via the alkyl compounds thereof, such astrimethylaluminum, triethylaluminum, trimethylgallium, triethylgallium,trimethylindium, triethylthallium, triphenylbismuth. Thallium may alsobe complexed in the form of cyclopentadienylthallium.

Dopant complexes with Sn or Pb are, for example, SnCp₂ and PbCp₂, or thepermethylated or perphenylated derivatives thereof, such as Pb(alkyl,aryl)₄, Sn(alkyl, aryl)₄ where, for example, alkyl=ethyl andaryl=phenyl.

Dopant complexes with As, Sb and Bi may be As(III), Sb(III), Bi(III)with alkyl or aryl ligands and mixed alkyl-hydrogen compounds such asarsine, stibine or bismuthine.

Dopant complexes with lanthanoids, for example Ce, Er, Gd, Hf, La, Nd,Pr, Sm, Tb, Tm and Yb, are, for example, cyclopentadienyl compounds andderivatives thereof such as tris(cyclopentadienyl)cerium,tris(cyclopentadienyl)erbium, tris(cyclopentadienyl)gadolinium,bis(cyclopentadienyl)dimethylhafnium, tris(cyclopentadienyl)lanthanum,tris(cyclopentadienyl)neodymium, tris(cyclopentadienyl)praseodymium,tris(cyclopentadienyl)samarium, tris(i-propylcyclopentadienyl)terbium,tris(cyclopentadienyl)thulium and tris(cyclopentadienyl)ytterbium.

In the process, it is additionally possible in process step C) for thedopant complex to be continuously evaporated and decomposed. Thedecomposition of the dopant complex can be performed by a methodselected from thermal heating with, for example, jets and/or wires,electromagnetic irradiation, for example with lasers, UV or IR, which ismatched to the absorption spectra of the dopant complexes used,irradiation with radiofrequencies or microwave irradiation, for exampleplasmas with the aid of carrier gases. The decomposition of the dopantcomplex may additionally take place in the gas phase.

It is thus possible to use dopants such as metals, which frequently havevery high melting points and are therefore difficult to evaporate, bythe use of a decomposable complex of the metals with a volatilecompound, such as the abovementioned ligands. The dopant complex servesas a precursor for controlled provision of the dopant, for examplesingle metal atoms or metal clusters. After the elimination of theligands, which may take place in the gas phase, the individual metalatoms or the metal clusters prepared in a controlled manner are present.The metal atoms and/or clusters do not coagulate since, under theproduction conditions, for example as a result of the application of areduced pressure of <10⁻⁴ mbar, the mean free path length is greaterthan the apparatus dimensions. The pure metals and/or metal clusters cantherefore be intercalated into the matrix materials before there is anycollision and hence cluster formation with further metal atoms.

In addition, in the process, a dopant which forms complexes with thematrix material in the course of intercalation into the matrix materialcan be used in process step C). The dopant can thus p- or n-dope thematrix material. When, for example, a matrix material and metal atomsand/or clusters are thus simultaneously applied to a substrate by vapordeposition, the metal atoms and/or clusters, once the ligands have beeneliminated, can be complexed by the matrix. This can formthermodynamically stable complexes of the particular metal atoms and/orclusters. For example, in the case of Fe as the dopant, an octahedralcomplex can form with an interaction with three matrix molecules (scheme1):

Scheme 1 shows a schematic of a symbolic matrix material with twonitrogen atoms which can coordinate to a metal Me. For example, thematrix material used may be Bphen or BCP, and the metal Me used Fe orCr, in which case three matrix molecules complex the metal via thenitrogen atoms thereof. As a result of the complex formation, the metalsare incorporated into the matrix material in a fixed manner and can nolonger diffuse within the matrix material.

If an organic semiconductive layer is to be n-doped with a dopant, it ispossible by way of example to determine whether free electrons areavailable for n-conduction after the introduction of the dopant into thematrix material. To this end, the electrons on the particular metal atomare counted; in the case of Fe, there are, for example, eight outerelectrons. The three illustrative ligands shown in scheme 1 have 3×4=12electrons available. In the matrix material, the iron atom is thus in anenvironment of twenty electrons. An electronically stable configurationconsists, however, of eighteen electrons. The two excess electrons arenow available as charge carriers for electron conduction. The matrixmaterial is thus n-doped.

An analogous calculation for Cr would not give any excess electrons forcharge transport. However, the metal Cr is so low in the electrochemicalseries (−0.56 V) that at least a partial charge transfer to the matrixis to be expected. Analogous calculations also apply to metals such asRu, for example. A complex of Ru with, for example, a bipyridyl matrixwould likewise provide two electrons for n-doping.

In general, the denticity of the matrix is not restricted to two. Higherdenticities increase the complex stability. Moreover, this calculationshould be understood as a model.

It is merely important that a net charge is available for electronconduction. For example, iron or chromium atoms can also enter into a πbond via two arene ligands.

According to the matrix material and dopant, the environment of thedopant, for example of a metal atom, may vary.

It is thus possible for linear, tetrahedral, octahedral or trigonalbipyramidal complexes with the matrix material to arise. For example,copper complexes with phenanthroline in a tetrahedral arrangement. Thesame considerations also apply for metal clusters which are prepared ina controlled manner and consist of two, three or more metal atoms.

The invention further relates to a process for producing anoptoelectronic device. The device has a substrate, a first electrode onthe substrate, which in operation releases charge carriers of a firstcharge, a first charge transport layer which transports charge carriersof the first charge, at least one emission layer on the first chargetransport layer, and a second electrode on the at least one emissionlayer, which in operation releases charge carriers of a second charge.In the process, the first charge transport layer has been produced by aprocess according to the details given above.

It is therefore possible to produce an optoelectronic device, forexample an organic light-emitting diode, which has at least one chargetransport layer which, owing to the doping, has increased charge carrierinjection. The doping may, for example, be n-doping. In that case, thefirst electrode is an electron-injecting cathode and the chargetransport layer is an electron transport layer. The above-describedprocess can achieve homogeneous doping of the charge transport layer,which results in an increased lifetime of the device.

The invention will be illustrated in detail with reference to thefigures:

FIG. 1 shows the schematic side view of an organic light-emitting diode.

FIG. 2 shows a schematic of an illustrative device for producing a dopedorganic semiconductive layer.

FIG. 1 shows the schematic side view of an organic light-emitting diodewith a substrate 10, a first electrode 20, a first charge transportlayer 30, an emission layer 40, and a second electrode 50. The chargetransport layer 30 has a doping of the matrix material, and has beenproduced by one of the processes described above with the matrixmaterials and dopants mentioned there. If required, such a layer is alsouseable for other components. An organic light-emitting diode mayadditionally have a second charge transport layer and/or a plurality ofemission layers (not shown here).

FIG. 2 shows a schematic of an illustrative device for producing a dopedorganic semiconductive layer. Within a vacuum recipient 60 is a rotatingsubstrate plate 70, on which a dopant complex is applied by vapordeposition to the substrate plate from an electrically heated ceramicnozzle 80 (indicated by dotted arrows). In addition, matrix material isapplied by vapor deposition to the substrate plate from a current-heatedmolybdenum boat 90 (indicated by dashed arrows). This makes it possibleto produce, on the substrate plate 70, an organic semiconductive layerwhich has a matrix material with homogeneously intercalated dopant.

In a working example, according to FIG. 2, Fe(CO)₅ is heated to 110° C.in a previously evacuated pressure vessel.

The internal pressure rises to about 1 bar. The evaporated dopantcomplex is passed at a passage rate of approx. 20 sccm through anelectrically heated, white-hot ceramic nozzle 80 into the vacuumrecipient 60. The nozzle points obliquely toward the rotating substrate70. A similarly mounted current-heated molybdenum boat 90 serves todeposit a matrix material, for example BCP. The current through themolybdenum boat is adjusted such that a growth rate of 0.1 nm/s isestablished. In this way, 30 nm of iron-doped BCP are prepared withinapproximately five minutes.

When, for example, Ni(CO)₄ is used in place of Fe(CO)₅, the pressurevessel is heated only to 40° C. This affords a nickel-doped layer.

It is additionally possible to use triethylaluminum in place of Fe(CO)₅.The pressure vessel is then heated to 80° C.

The Fe(CO)₅ can also be passed through a cold nozzle. In that case, alaser source is focused one centimeter above the inlet, which is matchedto the absorption pressure frequency of 2200 to 1700 cm⁻¹ of the IRcarbonyl bands of the Fe(CO)₅ and can thus decompose the dopant complex(the laser source is not shown in FIG. 2).

It is additionally possible to introduce solid Cr(CO)₆ into acurrent-controlled source and to decompose it analogously to theexamples cited above.

Instead of Cr(CO)₆, it is also possible to use dibenzenechromium. Asufficiently strong red laser can be used to eliminate the areneligands.

The embodiments shown in FIGS. 1 and 2 and the working examples can bevaried as desired. It should additionally be considered that theinvention is not restricted to the examples, but permits furtherconfigurations not detailed here.

1. A process for producing a doped organic semiconductive layer,comprising the steps of: A) providing a matrix material; B) providing adopant complex; and C) simultaneously applying the matrix material andthe dopant complex to a substrate by vapor deposition, wherein, inprocess step C), the dopant complex is decomposed and the pure dopant isintercalated into the matrix material.
 2. The process as claimed inclaim 1, wherein, in process step A), a matrix material which isselected from a group comprising phenanthroline derivatives, imidazolederivatives, triazole derivatives, oxadiazole derivatives,phenyl-containing compounds, compounds with fused aromatics,carbazole-containing compounds, fluorene derivatives, spirofluorenederivatives and pyridine-containing compounds is provided.
 3. Theprocess as claimed in claim 1, wherein, in process step C), the dopantcomplex is decomposed to a dopant and at least one ligand.
 4. Theprocess as claimed in claim 3, wherein the dopant comprises metalsand/or metal clusters.
 5. The process as claimed in claim 4, wherein themetals and/or metal clusters are selected from a group comprisingtransition metals, lanthanoids and metals of the main groups.
 6. Theprocess as claimed in claim 3, wherein the at least one ligand isselected from a group comprising carbonyl, phosphine, cyclopentadienyland arene ligands.
 7. The process as claimed in claim 1, wherein, inprocess step C), the dopant complex is continuously evaporated anddecomposed.
 8. The process as claimed in claim 1, wherein, in processstep C), the dopant complex is decomposed by a method selected fromthermal heating, electromagnetic irradiation, irradiation withradiofrequencies and microwave irradiation.
 9. The process as claimed inclaim 8, wherein the decomposition of the dopant complex takes place inthe gas phase.
 10. The process as claimed in claim 1, wherein, inprocess step C), the pure dopant forms complexes with the matrixmaterial in the course of intercalation into the matrix material. 11.The process as claimed in claim 1, wherein the dopant n-dopes the matrixmaterial.
 12. A process for producing an optoelectronic device whichcomprises a substrate, a first electrode on the substrate, which inoperation releases charge carriers of a first charge, a first chargetransport layer which transports charge carriers of the first charge, atleast one emission layer on the first charge transport layer, and asecond electrode on the at least one emission layer, which in operationreleases charge carriers of a second charge, wherein the first chargetransport layer has been produced by a process as claimed in claim 1.